1. Technical Field
The present invention relates to a development roller and an image forming apparatus including the development roller.
2. Description of Related Art
In general, in an electrophotographic image forming apparatus, toner is supplied to an electrostatic latent image formed on an image carrier to prepare a toner image, and the toner image is transferred to a paper to obtain a recorded image.
As a unit that supplies the toner to the electrostatic latent image, a development roller is used. The development roller includes a development sleeve formed in a cylindrical shape and made of a non-magnetic material, and a magnet roller housed in the development sleeve and forming a magnetic field to cause toner napping to be generated on a surface of the development sleeve.
The magnet roller normally includes a magnet portion forming a magnetic field and a shaft portion adapted to support the magnet portion in the development sleeve.
In such an image forming apparatus, it is important to arrange the magnet roller at an accurate position with respect to the image carrier for achievement of a good recorded image.
To this end is developed the magnet roller in which the magnet portion and the shaft portion are formed integrally with use of an equal material to prevent relative positions of the magnet portion and the shaft portion from being misaligned.
Japanese Patent Application Laid-Open (JP-A) No. 2000-114031 describes such a magnet roller in which the magnet portion and the shaft portion are formed integrally, in which the magnet portion is formed so that end portions thereof may be gradually thinned. The reason for thinning the end portions of the magnet portion in this manner is to weaken magnetic forces generated at the end portions to cause a magnetic force distribution in an axial direction to be uniform (even).
On the other hand, as for a magnetic force distribution in a circumferential direction of the magnet roller, toner napping cannot be generated on the surface of the development sleeve without an intensity variation in the magnetic force.
In a case where the end portions of the magnet portion are thinned even under such circumstances, the magnetic force distribution in the circumferential direction will be broad (a state in which the intensity variation in the magnetic force is not apparent). This causes a problem in which the magnetic force cannot be maximum at a position contacting the image carrier, and in which a good recorded image cannot be obtained.